The proposed research project is a first submission of an R01 application by a young investigator. The project draws directly upon developmental theories of substance use and conduct problems to identify important research questions that are currently difficult to evaluate empirically and would greatly benefit from the development of new quantitative methodologies. The proposal is organized around three major goals. First, a new class of innovative longitudinal models of individual differences in change over time will be developed with the explicit purpose of allowing powerful, flexible and dynamic empirical tests of developmental trajectories of substance use. These new models will allow for the estimation of a variety of multivariate latent curve models that are more robust to non-normality and model misspecification, that incorporate two- and three-way interactions among latent growth factors in the prediction of later problem behaviors, and that simultaneously examine characteristics of multiple developmental trajectories before and after important life transitions. Second, the validity and utility of these new quantitative methods will be closely evaluated with particular emphasis on future applications in studies of substance use over time. Evaluation of these new models will primarily be accomplished through the use of comprehensive computer simulations that will focus on finite sampling characteristics that are commonly encountered in applied longitudinal research of substance use and abuse including small sample size, multivariate non-normality, and model misspecification. Third, these new models will be applied to a high quality existing longitudinal data set to examine the course, causes and consequences of co-occurring developmental trajectories of conduct problems and substance use in a large sample of adolescents. Data will be drawn from an existing longitudinal study of over 1000 adolescents ranging in age from 9 to 18 assessed once a year for four years. Application of the newly developed quantitative, methods to this data will allow for a detailed evaluation of a number of theoretically derived research hypotheses including questions about reciprocal developmental relations between substance use and conduct problems over time, interactions between trajectories of conduct problems and substance use in the prediction of later problem behavior, and the effects of school and pubertal transitions on these developmental trajectories. The empirical applications will also serve to demonstrate and disseminate the newly developed models to a wide audience of applied substance use researchers. The unifying goal of the proposed project is to focus extended effort on explicitly linking the fields of quantitative methodology and applied developmental psychopathology to create and disseminate a new class of innovative statistical models that are optimally suited for studying individual differences in developmental trajectories of substance use over time.